The present invention relates to the non-destructive evaluation of the condition of a material, and more particularly to in situ non-destructive evaluation which is capable of determining the condition of a material by measuring its specific damping capacity, or internal friction damping (IFD).
Knowledge of the specific damping capacity of a material has a variety of applications. For example, it is used to determine the authenticity of coins, the soundness of castings, the operating condition of railroad wheels and the quality of musical instruments and glassware. Furthermore, it is useful as a research tool in physical metallurgy, vibration control of high-speed vehicles, metal fatique and the study of properties of metals and alloys. The present invention is particularly concerned with, but not limited to, the utilization of the specific damping capacity of a material as a means of detecting and monitoring flaws in objects and predicting their useful life.
In the past, the specific damping capacity of a material has been determined by freely suspending an object, vibrating the object at its resonant frequency, and measuring the amplitude of the vibrations. This technique is disclosed in U.S. Pat. Nos. 3,592,050, issued to Nutt, Jr., et al on July 13, 1971, and 3,623,358, issued to Sugimoto on Nov. 30, 1971.
As is well known, when an object is vibrated at its resonant frequency, the amplitude of the vibrations is at a maximum with respect to vibration at any other frequency for a given vibration inducing input force. This can result in relatively high amplitude vibrations which produce high level stresses on the object being evaluated. Since the stress vs. strain curve for a material is normally not linear throughout the stress range, high level stress imparted to an object produces an output signal which is not always linearly proportional to the input force.
In the prior art systems such as those disclosed in the previously mentioned patents, the object being tested is freely suspended by hanging the object with the use of wires or the like or by placing the object on a support which acts as a fulcrum, so that the object is free to vibrate at its resonant frequency. This technique is not practical for the evaluation of structural components located in situ. For example, where a boiler tank is to be tested for its structural soundness after a period of use, it is necessary to disconnect the tank from the remainder of the system into which it is incorporated, remove it from its support and suspend it so that it can freely vibrate. This means that the system must be shut down while the test is being performed, resulting in lost time and probably economic loss.
Furthermore, where an object is suspended and vibrated, it is only possible to test the structural integrity or composition of the object as a whole. It is not possible to individually test non-separable portions of the object or to determine the location of detected flaws with a resonable degree of precision.
In the prior art non-destructive evaluation systems, the results of the measurements obtained during testing were used only to determine the material composition or structural condition of the tested object. They were not utilized to obtain other useful information such as the life expectancy of an object or to monitor conditions of the object such as crack growth rate.